State-of-the-art cellular radio communications systems such as the Global System for Mobile Communications (GSM) and the Universal Mobile Telecommunications System (UMTS) provide radio coverage for a plurality of mobile stations by placing a plurality of base stations in a substantially regular arrangement across an area that is to be covered by said radio communications system. Each of said base station then defines a cell of said radio communications system and uses a set of transmission channels, which may for instance be defined by frequency carriers, spreading codes or time slots, to allow for data transmissions between said base station and said mobile stations that are located in said cell. To reduce interference between data transmissions of neighboring cells, orthogonal sets of transmission channels are used by base stations of adjacent cells, which is for instance achieved by defining that sets of transmission channels of neighboring cells use different frequency bands. The overall available frequency bandwidth then is split into frequency bands, the number of which is denoted as cluster size, and the frequency bands then are assigned to the base stations of the radio communications system so that a maximum distance between base stations using the same frequency bands is achieved.
However, the centered position of the base station in the middle of a substantially circular cell (which is approximated as hexagonal cell to allow for a seamless paving of the coverage area) leads to a decrease of the Carrier-to-Interference (C/I) power ratio towards the cell border, which is mainly due to the decrease of the power of an electromagnetic wave being proportional to the propagation distance raised to the power of a path loss exponent, which typically is larger than 2. As in all state-of-the-art transmission technologies, the end-to-end throughput between the base station and the mobile station is linked to the C/I, correspondingly the end-to-end throughput declines towards the cell border, which makes it difficult to guarantee a certain end-to-end-throughput for mobile stations that can be located anywhere in a cell or may even be moving through the cell.
Modern radio air interfaces have available various physical modes (PHY-modes), i.e. different combinations of modulation and coding schemes that are each applicable up to a minimum C/I signal value at a receiver in the cell. A high-valued PHY-mode is transmitting symbols with a high number of bits per symbol whilst a low-valued PHY-mode is just transmitting binary symbols. A mobile station close to the base station typically experiences a high C/I value and therefore can make use of a high-valued PHY-mode whilst a mobile station close to the cell border experiences typically a low C/I value and therefore preferably is assigned a low-valued PHY-mode. This situation is called the “unfairness in transmission rate assignment” to mobile stations, dependent on their location in the cell.
CDMA-based cellular systems allow to partly overcome this unfairness on cost of the whole capacity available in a cell: there the mobile stations close to the cell border could be served with a comparably higher transmission rate as mobile stations close to the base station by increasing the amount of power of the transmission channels of said mobile stations close to the border, at the cost of a reduction of the total cell capacity that can be provided to other users and a correspondingly substantially reduced spectral efficiency.
The fact that the area of a circle increases quadratically with its radius, leads to the situation that most mobile stations are located near the cell border, when it is assumed that the mobile stations are equally distributed over the cell area. Consequently, a substantial portion of the mobile stations in a cell suffer from a low end-to-end throughput (or cause reduction of the overall cell capacity in a CDMA system), directly affecting the spectral efficiency (in bit/s/Hz/m2) that can be achieved with such a cellular radio communications system, and rendering the application of this cellular concept disadvantageous with respect to the requirements of future mobile radio communications systems.
Prior art document “Power Consumption reduction by multi-hop transmission in cellular networks” by Jee-Young Song et al., IEEE 60th Vehicular Technology Conference (VTC 2004-Fall), Sep. 26-29, 2004, Los Angeles, Calif., USA, pages 3120-3124, discloses relaying in cellular networks, wherein two-hop transmissions take place between a base station and a mobile station via a relay station. The relaying functionality is provided by mobile stations of the network, i.e. the relay stations are mobile stations. This prior art document concentrates on the issue of power consumption. It is assumed that each mobile station and base station has limited transmission power. Based on this assumption, closed-form solutions for the probability that a mobile station is in range of a (mobile) relay station (which in turn is in range of the base station) are derived. Furthermore, the transmission power of single-hop and two-hop transmission between base and mobile stations are derived and compared, yielding the result that, as the number of (mobile) relay stations increases, the probability that mobile stations find a (mobile) relay station and save power increases.
Prior art document “Capacity of a Relaying Infrastructure for Broadband Radio Coverage of Urban Areas” by Tim Irnich et. al, IEEE 58th Vehicular Technology Conference (VTC 2003-Fall), Oct. 6-9, 2003, Orlando, Fla., USA, vol. 5, pages 2886-2890, discoses the introduction of relaying into cellular broadband radio systems in urban areas to improve coverage. A methodology to quantify the influence of relaying on the capacity of a single base station is presented. Therein, a base station with four fixed relay stations covering the same area like five base stations in a conventional cellular architecture is considered.